This paper describes the specific features of supercritical hydrothermal synthesis method for organic modified nanoparticles synthesis and their applications for hybrid materials with polymers. Under the supercritical state, water-organic molecules-inorganic precursors form a homogeneous phase with high concentration, and the water molecule works as an acid or base catalyst. This leads to form optimum conditions for synthesizing organic modified nanoparticles with high concentrations. Also, this method can control the exposed surface, which leads to fabricate a new active catalyst. The formed nanoparticles can be mixed with organic solvents or with polymers, and thus, this method can fabricate new hybrid polymers.

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  In this paper, we introduce novel findings that we have obtained through the several experimental techniques with high-voltage pulsed power in high-density fluids (such as subcritical water, supercritical carbon dioxide or supercritical argon). In laser ablation in supercritical carbon dioxide, we succeeded in the synthesis of gold nanoparticles from gold substrate. Next, we were able to produce hard amorphous carbon thin film from typical organic chemicals such as phenol and aniline. Furthermore, by using a novel technique “electro-spinning in supercritical carbon dioxide”, we succeeded to prepare unique morphology fibers such as hollow or balloon-like structures in one step.

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  We describe particle coating using ultrasonic irradiation in high-pressure carbon dioxide (CO₂). Host microparticles were coated with guest nanoparticles. Shock waves generated by collapsing cavitation bubbles induced with ultrasonic irradiation accelerated the deagglomeration and dispersion of guest nanoparticles, and achieved dry particle coating in liquid CO₂. Host microparticles coated with guest nanoparticles were observed using scanning electron microscopy (SEM). This revealed that the guest particles were completely coated with guest nanoparticles, and a smooth coating surface was obtained.

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  In this article, recent advances in development of continuous production method of high crystallinity metal oxide nanoparticles (NPs) through hydrothermal synthesis using micromixers and high-temperature and high-pressure water are summarized. This method has been attracting much attention as an innovative production technology of desired amount of NPs having desired particle properties for reducing development period of products using NPs and increasing application fields of NPs. Fundamental principle of this method, optimization of mixing condition, reaction rate and equilibrium of hydrothermal synthesis, and advantages of developed micromixers are described.

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  Single-phase super-hard polycrystalline cBN and diamond have been synthesized by direct conversion sintering from hBN and graphite, respectively, under static high pressure and high temperature. The polycrystalline cBN and diamond consist of fine grains of several tens or hundreds nano-meters, having high hardnesses which are higher than those of their single crystals. They also have high transverse rupture strengths (TRSs) which are much higher than those of conventional sintered compacts containing binder materials. The fine microstructure features without any secondary phases and surpassing mechanical properties of the single-phase super-hard polycrystalline cBN and diamond are promising for applications in next-generation high-precision and high-efficiency cutting tools.

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  Indium tin oxide (ITO) nanoparticles have been synthesized from the mixed ethylene glycol solution of indium (III) chloride, tin (II) chloride and sodium hydroxide by the solvothermal process with the addition of oleylamine or oleic acid as a protective reagent. The x-ray diffraction patterns of particles synthesized at 250°C show that the ITO begins to be produced after the reaction duration of 6 h when oleylamine is added to the reaction solution, indicating a promoting effect on the ITO formation, whereas only indium oxyhydroxide is formed when oleic acid is used. The transmission electron microscopic observation has revealed that ITO nanoparticles obtained by the addition of oleylamine are monodisperse and their size increases from 4-5 nm to 50-60 nm with elongating reaction duration from 6 to 96 h. ITO thin films are prepared by the multiple deposition of a collodion membrane incorporating synthesized ITO nanoparticles on the glass substrate followed by sintering at 400-500°C. They exhibit a good transparency in the visible region and a specific resistivity as low as 1.0 Ω·cm.

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  Development of DAC and related basic technologies are described, including a high-pressure gas-loading apparatus. Pressure-induced phase transitions and structural change of the elements under high pressure are reviewed, focusing on the molecular dissociation of iodine and electronic topological transition in zinc. Importance of hydrostaticity for the determination of accurate equation of states is discussed.

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  Angle dispersive powder x-ray diffraction experiments using a flat imaging plate (IP) are one of the most popular methods to study high-pressure material science. An intensity distribution on Debye rings involves information of the number of crystallites, lattice preferred orientation, and lattice strains under stress. The developed computer software by the author simulates the Debye-ring pattern with given parameters including above ones and makes a 2D whole pattern fitting to observed images.

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  The present review introduces our recent studies of the title subjects that have led to the JSHPST Award for Young Scientists, 2011. We studied the structure, dynamics, and reactions in water under high-pressure and high-temperature conditions using NMR and computational methods. The NMR apparatus was newly developed and was applied for the self-diffusion measurements of water and nonpolar organic fluids over a wide range of pressure and temperature. Molecular dynamics simulation combined with a new analysis scheme provided new insights into the translational dynamics in the supercritical water in the medium-density region in relation to the structure and lifetime of the solvation shell.

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  Pressure range of in situ x-ray observation has been extended very close to 100 GPa, by interfacing the Kawai-type multi-anvil apparatus (KMA) equipped with sintered diamond anvils with the synchrotron radiation at SPring-8. High-pressure phase equilibria in Fe₂O₃ and MgGeO₃ are summarized. Isostructural reduction in electrical resistance of hematite by three orders of magnitude at pressures 54-56 GPa strongly suggests the occurrence of a Mott transition. In MgGeO₃, special attention was paid to the dP/dT slope of the phase boundary between perovskite and post-perovskite phases. Spin transition of Fe²⁺ in (Mg_0.83Fe_0.17)O is also briefly reported.

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